1. Field of the Invention
The present invention relates to a position detecting apparatus configured to detect a position of a movable member, and to a lens apparatus and an image pickup apparatus including the position detecting apparatus.
2. Description of the Related Art
As an apparatus for measuring a distance of movement of an object, besides an incremental encoder for measuring a distance of relative movement, there is hitherto known an absolute encoder capable of detecting an absolute position.
Japanese Patent Application Laid-open No. H08-304113 discloses a Vernier type absolute encoder. The Vernier type absolute encoder disclosed in Japanese Patent Application Laid-open No. H08-304113 has a configuration including at least two grid patterns having different pitches on a track. Based on a slight shift between detected signals, which is generated due to a difference in pitch between the grid patterns, a position in a section over which the Vernier type absolute encoder circulates once is identified (hereinafter also referred to as “absolute position detection” or “absolute position calculation”).
Japanese Patent Application Laid-open No. H05-45151 discloses timing of switching to an absolute counting operation in the Vernier type absolute encoder. The Vernier type absolute encoder has a configuration in which track patterns are formed at smaller pitches and at larger pitches. As the timing of switching to the absolute counting operation based on the tracks described above, when a speed is lowered to a speed at which erroneous detection does not occur in an incremental measurement at the smaller pitches, an operation of the Vernier type absolute encoder is switched to the absolute counting operation.
However, the absolute encoder disclosed in Japanese Patent Application Laid-open No. H08-304113 has the following problem. In the case where motes and scratches are present on a scale, an absolute position is calculated based on an erroneous detection signal. As a result, there arises a problem in that a correct absolute position cannot be calculated.
Further, the absolute encoder disclosed in Japanese Patent Application Laid-open No. H05-45151 has the following problem. In the case where motes and scratches are present on a scale at a position at which the speed is lowered so that erroneous detection does not occur in incremental measurement with fine pitches, an erroneous detection signal is detected. As a result, an absolute position is also calculated based on the erroneous detection signal.